Add String.prototype.includes polyfill

Summary:
Add a polyfill for `String.prototype.includes` taken from MDN (https://developer.mozilla.org/en/docs/Web/JavaScript/Reference/Global_Objects/String/includes)
See https://github.com/facebook/react-native/issues/5727
Closes https://github.com/facebook/react-native/pull/5735

Reviewed By: svcscm

Differential Revision: D2906667

Pulled By: vjeux

fb-gh-sync-id: e02f605bf57171062b29a98b98ba9fc898cedfc2

packager/react-packager/src/Resolver/polyfills/String.prototype.es6.js

@@ -83,3 +83,18 @@ if (!String.prototype.repeat) {
     return result;
   };
 }
+
+if (!String.prototype.includes) {
+  String.prototype.includes = function(search, start) {
+    'use strict';
+    if (typeof start !== 'number') {
+      start = 0;
+    }
+    
+    if (start + search.length > this.length) {
+      return false;
+    } else {
+      return this.indexOf(search, start) !== -1;
+    }
+  };
+}